1. Field of the Invention
The present invention relates generally to an apparatus and method for controlling a drop of a requested call in a wireless network. More particularly, the present invention relates to an apparatus and method for guaranteeing a quality of service (QoS) by keeping a dropping probability of a handoff call requested without user mobility information below a predefined level.
2. Description of the Related Art
Recently, the population of mobile users, particularly among those using systems such as CDMA (Code Division Multiple Access), PCS (Personal Communication System), and GSM (Global System for Mobile communication), has grown at a rapid rate. Also, the demand has increased for multimedia applications requiring high bandwidth, such as high quality video. Therefore, the current trend in wireless networks is to decrease cell sizes (into micro-cells or pico-cells) to provide higher capacity and accommodate more users in a given area. However, smaller cell sizes increase the frequency of handoffs, which results in rapid changes in the conditions of the network traffic. Thus, the QoS provisioning in wireless networks becomes more difficult to achieve.
One of the important QoS issues in wireless networks is how to control handoff drops. When a mobile user moves into an adjacent cell during a session, a handoff occurs and the mobile user communicates continuously through the new base station (BS). However, the handoff could fail if the available bandwidth in the new cell is insufficient for the mobile user. Such a failure is referred to as “a handoff drop”.
In general, a handoff drop is considered to have more of a negative impact on a users' perception of quality than does a new call block. As a result, there are required strategies for prioritizing handoff calls over new calls, for example, such as reserving a portion bandwidth exclusively for handoffs. The concept of the bandwidth reservation for handoffs was first introduced in mid-80s (see D. Hong and S. S. Rappaport, “Traffic model and performance analysis of cellular radio telephone systems with prioritized and non-prioritized handoff procedures,” IEEE Trans. on Vehicular Technology, 35(3), August 1986). Since then, various strategies that give priority to handoffs have been studied.
Ideally, it would be desirable if handoff drops never occur. However, this would require the network to reserve so much bandwidth in all cells in which a particular mobile unit might enter, and would result in very low channel utilization and/or high new call blocking probability Pb. Therefore, another approach is to provide probabilistic QoS guarantees by keeping the handoff dropping probability Pd below a certain level, rather than eliminate it completely.
In a paper, M. Naghshineh and M. Schwartz, “Distributed Call Admission Control in Mobile/Wireless Networks,” IEEE Journal on Selected Areas in Communications, 14(4), May 1996, the admission threshold to satisfy the QoS constraint is a calculation based on the number of users in the current cell and adjacent cells, given the probability that a mobile unit would be handed-off within some time interval. However, this scheme does not specify how to predict the user mobility, and such predictability is essential in their scheme, and assumes the exponential distribution of the cell residence time.
In practice, the cell residence time may not be exponentially distributed (see M. M. Zonoozi and P. Dassanayake, “User Mobility Modeling and Characterization of Mobility Patterns,” IEEE Journal on Selected Areas in Communications, 15(7), September 1997).
Moreover, another paper, authored by O. T. W. Yu and V. C. M. Leung, “Adaptive Resource Allocation for Prioritized Call Admission Over an ATM-based Wireless PCN,” IEEE Journal on Selected Areas in Communications, 15(7), September 1997, also proposes a technique to compute the reserved bandwidth to maintain the handoff dropping probability Pd within a specified level. However, the paper also assumes the exponentially distributed cell residence time.
In yet another paper, D. Levine, I. Akyildiz, and M. Naghshineh, “A Resource Estimation and Call Admission Algorithm for Wireless Multimedia Networks Using the Shadow Cluster Concept,” IEEE/ACM Trans. on Networking, 5(1), February 1997, the shadow cluster concept has been used as a way to estimate future resource requirements and perform admission control in order to limit the handoff dropping probability Pd. In this scheme, mobiles inform the base stations (BSs) in neighboring cells of their bandwidth requirements and movement patterns at the call setup time. Based on this set of information, the BSs predict future demands and admit only those mobiles that can be supported adequately. The drawback of this particular scheme is that precise user mobility should be known a priori, which is impractical, and it requires the exchange of a large number of messages among BSs.
A more practical method of predicting the user mobility has been presented quite recently in a paper, by S. Choi and K. G. Shin, “Predictive and Adaptive Bandwidth Reservation for Handoffs in QoS-Sensitive Cellular Networks,” in Proc. ACM SIGCOMM'98, pp. 155-166, September 1998. This method uses the adaptive and predictive bandwidth reservation scheme to provide probabilistic QoS guarantees. Herein, the method disclosed in this paper will be referred to as “CS98”. This CS98 scheme is based on the observed history of mobility information to calculate the reserved bandwidth.
In CS98, a predictive and adaptive bandwidth reservation scheme is proposed. First, user mobility is estimated based on an aggregate history of handoffs observed in each cell. This user mobility information is then used to (probabilistically) predict mobiles' moving directions and handoff times. For each cell, the bandwidth to be reserved for handoffs is calculated by estimating the total sum of fractional bandwidths of the expected handoffs within a estimation time window Test. The estimation time window Test is adaptively controlled for the efficient use of bandwidth and effective response to (i) time-varying traffic/mobility and (ii) inaccuracy of mobility estimation.
Although the CS98 scheme is not based on any impractical assumptions, it has still high complexity (see S. Choi and K. G. Shin, “Comparison of Connection Admission Control Schemes in the Presence of Handoffs in Cellular Networks,” in Proc. ACM/IEEE Mobicom'98, October 1998). This is because, in the CS98 scheme, a rather complex history-based method is used to calculate the target reserved bandwidth. Handoff events must be cached and the handoff probability of every call in adjacent cells must be calculated every time a new call is tested for admission.
Here, it should be noted that many previous schemes were based on the user mobility information, called “mobile-oriented reservation schemes”. If the design goal is to reduce handoff drops to as little as possible, then the user mobility information should be used to predict mobiles' handoff times and next cells and to reserve some bandwidths for those mobile users. However, if the design goal is to keep the handoff dropping probability Pd below a certain level, the user mobility information is not essential. It should be noted that a handoff drop is basically a cell-oriented event (i.e., a handoff drop occurs when a cell is overloaded).
There is another practical scheme in which the cell-oriented policy was introduced (see C. Oliveira, J. B. Kim and T. Suda, “An Adaptive Bandwidth Reservation Scheme for High-Speed Multimedia Wireless Networks,” IEEE Journal on Selected Areas in Communications, 16(6), August 1998). Herein, this scheme will be referred to as “OKS98 scheme”. This OKS98 scheme determines the amount of reserved bandwidth by the largest of all the requested bandwidths from adjacent cells. After some bandwidth is reserved, it is dynamically adjusted at each cell to keep the handoff dropping probability Pd below a target value.
However, this original adaptive algorithm in OKS98 has some un-specified aspects. First, there is no mention about the monitored period of the handoff dropping probability Pd. Second, it is not clear when to increase or decrease R. In addition, this OKS98 scheme has the inter-cell unfairness problem.